In physics class, the girl to boy ratio is $3$ to $4$. If there are a total of $35$ students, how many girls are there?
Solution: A ratio of $3$ girls to $4$ boys means that a set of $7$ students will have $3$ girls and $4$ boys. A class of $35$ students has $5$ sets of $7$ students. Because we know that there are $3$ girls in each set of $7$ students, the class must have $5$ groups of $3$ girls each. There is a total of $15$ girls in physics class.